1. Field of the Invention
This invention relates to a method and device for image reduction of a read-out image outputted from an image reading means such as an image scanner, and more particularly to a method and device for allowing a read-out image obtained from a halftone image such as a photograph to be reduced to scale in accordance with a dither pattern and outputted to a monitor display of an image processing system, a printer of a wordprocessor or the like.
2. Description of the Prior Art
Not infrequently there are times when a read-out image obtained by using an image reading means such as an image scanner is reproduced on a reduced scale on a monitor display of an image processing system or the like. When a given objective image of 100mm in width is read out by use of an ordinary image scanner having a resolution of 8 dots/mm, a resultant read-out image represented in terms of binary bits is composed of picture elements of as many as 800 dots in the width direction. However, an ordinary monitor display such as a CRT which is widely applied to general purpose computers for image processing has generally a display field of a resolution of 640.times.400 picture elements. This means that the read-out image from the image scanner cannot be completely displayed on such a monitor display. It is therefore necessary to reduce a read-out image to be displayed on the monitor display. Therefore, an image-reducing process has been conventionally carried out by thinning out, for example, every other pixel to reduce the read-out image to a scale of one-half.
Thus, in order to reduce the read-out image to half the originally given image, every matrix with 2 rows and 2 lines (2.times.2 matrix) in the read-out image pattern may be simply reduced to 1.times.1 matrix (one pixel). In a case of reducing a high-contrast pattern such as a character which can easily be subjected to simple digitization processing, the visual impressions of the resultant image which is obtained as a result of subjecting a given character pattern to simple digitization processing and thinning-out processing is little removed from the original character image. However, in a case that a digitized image which is obtained from an originally given halftone image such as a photograph by being subjected to simple digitization processing is simply thinned out every other pixel to be reduced to half the original to obtain a reduced image, the reduced image thus obtained is not visually faithful to the original halftone image.
There is generally known a dither image-processing method for subjecting a given halftone image to image digitization. This dither image-processing method will be explained in brief with reference to the typical dither pattern model illustrated in FIG. 1(A). First, a given original image Io to be subjected to image processing is virtually defined in a matrix and the level of brightness at each picture element of the matrix of the original image thus defined is measured to obtain a brightness matrix pattern Mb. In the illustrated dither pattern model, the brightness is expressed in 16 gradient levels as illustrated in FIG. 1(B). Next, the level of brightness at each picture element of the brightness matrix pattern Mb is compared with threshold value prescribed in each pixel of a predetermined dither pattern D1. When the level of brightness at one picture element of the brightness matrix pattern Mb is lower than the threshold value of the corresponding pixel of the dither pattern D1, the picture element is regarded as being "black", thereby to obtain an intermediate dither image Id to be outputted to a monitor display or printer. This dither image processing method has been generally applied to image processing because it can reproduce the given halftone image with relatively high fidelity of image to the original.
In a case where the intermediate dither image Id obtained by the aforementioned processes is reduced to one-half, there have been so far used such image reducing methods as illustrated in FIGS. 2(A) to 2(C). According to these conventional methods, in its own way, the intermediate dither image Id can be reduced to one-half by reducing every 2.times.2 matrix to one picture element. In the image reducing method shown in FIG. 2(A), brightness feature in every picture element of a reduced resultant image Ir to be outputted to a monitor display or the like is determined on the basis of the brightness of one pixel at a specific location in a unit matrix. Namely, in the first model illustrated in FIG. 2(A), the upper and left picture elements (pixels a) in every unit matrix of 2.times.2 pixels are simply applied to the respective picture elements a' in the reduced image Ir. In the second model illustrated in FIG. 2(B), when the unit matrix with 2.times.2 pixels in the intermediate dither image Id has two or more black picture elements, the corresponding picture element in the reduced image Ir is determined as a "black" picture element. Conversely, when the unit matrix with 2.times.2 pixels of the intermediate dither image Id has two or more white picture elements, the corresponding picture element in the reduced image Ir may be determined as a "white" picture element, as illustrated in FIG. 2(C). However, as is apparent from the resulting patterns of the reduced images Ir, all the reduced images are wholly different in visual impressions from the given original images Io. Thus, the conventional image reducing methods could not reproduce a halftone image such as a photograph with high fidelity of image to the original and were not practical use.